PROJECT SUMMARY Current antidepressant drugs for major depressive disorder (MDD) require several weeks or even months of administration to achieve therapeutic effect. This delay for drug efficacy not only prolongs distress and impairment for depressed patients, but is also life threatening for suicidal MDD patients. This delay has established the well-accepted theory that therapeutic reversal of persistent pathological adaptations in MDD patients can only be achieved by chronic treatments. However, recent clinical studies such as deep brain stimulation, ketamine and scopolamine therapies have demonstrated the ability to regulate mood states within minutes or hours. These groundbreaking findings provide new hope to shorten the distress and minimize the risk of suicide for MDD patients. The main purpose of this application is to explore new drug targets for rapid MDD treatment. Early studies have implicated the midbrain dopamine system for rapid antidepressant treatment. Consistently, our recent findings and other studies show that optogenetically activating or inhibiting dopamine neurons in the ventral tegmental area (VTA) circuits can rapidly and bi-directionally regulate depression-related behaviors in rodent models of depression. Additionally, in a repeated social defeat stress (RSDS) model of depression, we have observed that pharmacological inhibition of VTA HCN (hyperpolarization-activated cyclic nucleotide-gated) channels reverse the pathophysiological hyperactivity of VTA dopamine neurons and achieved antidepressant-like effects within one hour. In our initial follow-up studies, we unexpectedly found that a single intra-VTA infusion of a HCN blocker induced rapid and long-lasting antidepressant-like effects. The single infusion-induced antidepressant efficacy lasted ~2 weeks. Similarly, a single systemic administration of this HCN blocker also induced rapid and ~2 weeks long antidepressant-like efficacy, which is very different from standard antidepressants. Focusing on the rapid and long-lasting treatment effect, the overall objectives of this application are: (1) to systematically define dose-dependent effects of three selected HCN blockers on VTA dopamine neuron activity and depressive behaviors; and (2) to determine the dopamine mechanisms that underlie the long-lasting antidepressant efficacy induced by single exposure to HCN blockers. These proposed studies will provide new HCN channel mechanisms for rapid and long-lasting depression treatment. Upon the completion of this project, we also expect novel information to improve our knowledge of dopamine circuit mechanisms of depression.